1. Technical Field
The present invention relates to electrophotographic image forming apparatuses, such as copiers and printers, and particularly, to a wet-type image forming apparatus that develops an electrostatic latent image using a liquid developer, such as liquid toner.
2. Related Art
As an electrophotographic printer, a line printer (image forming apparatus) is known. In a line printer, devices such as a charging device, a line printer head (line head), a developing device, and a transfer device are disposed adjacent to the periphery surface of a photosensitive drum on which exposure is to be conducted. Specifically, the charging device electrically charges the periphery surface of the photosensitive drum, and the line head selectively emits light from light-emitting elements thereof so as to conduct exposure on the periphery surface of the photosensitive drum. Thus, an electrostatic latent image is formed, which is subsequently developed using toner supplied by the developing device so that a toner image is formed. The transfer device then transfers this toner image onto a sheet.
Known light sources of the line head are light-emitting diodes (LED) and organic electroluminescence (EL) elements. In organic EL elements, a solid fluorescent material composed of an organic material is used as a light-emitting layer. Organic EL elements are light-emitting elements that produce light by means of electroluminescence. An organic EL element has an element portion with a thickness of 1 μm or less, and can be formed into a desired shape in accordance with the shape of the substrate. For this reason, organic EL elements are suitable for manufacturing compact line heads.
Since light emitted from an LED or an organic EL element diffuses, an image-formation optical system, like a rod lens array, is necessary to form the image of the light on a photosensitive drum. However, the aperture angle of light in a rod lens array is generally very small, which implies that a rod lens array can transmit light of only a specific angle. Therefore, especially in a case where the light source emits diffused light, the incident efficiency and transmission efficiency of light is extremely low, preventing the efficient use of the light from the light source. In a line head having LED as a light source, forming a desired latent image is possible by emitting an excessive amount of light from the LED. In contrast, in a printer head equipped with organic EL elements, emitting light from the light source by an excessive amount is not preferable since the organic EL elements have a short life. Consequently, in a case where organic EL elements are used as a light source, it is important that the light emitted from the organic EL elements be utilized efficiently.
JP-A-2004-195788 discloses an example of an image forming apparatus in which a light source is disposed inside a photosensitive drum, and the light source and the photosensitive drum have resin therebetween for preventing interfacial reflection. In this configuration, a rear exposure technique is applied in which a section subject to exposure is exposed to light from the inside of the photosensitive drum. This is problematic in view of cost and image quality since the configuration requires special components, such as a transparent base material for the photosensitive drum and transparent electrodes for the electrodes.